Shallow water mooring system using synthetic mooring lines

ABSTRACT

A shallow water mooring system applies to offshore industry for oil and gas exploration, and production. This mooring system employs non-steel synthetic lines to provide the desired flexibility for shallow water mooring system applications. Piled submerged towers at seabed serve as elevated anchors to keep the whole synthetic mooring lines off the seabed under environmental loading conditions. Other buoyant means such as a buoy is also utilized to further help the synthetic mooring lines off the seabed during their service life.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.60/595,682, “SHALLOW WATER MOORING SYSTEM USING SYNTHETIC MOORINGLINES,” filed Jul. 21, 2005, and hereby incorporated by reference.

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates generally to a shallow water mooring system.

2. Background Art

Mooring systems utilizing steel wire and steel chains for offshoremoored floating vessels have been used for many years. Most vessels arelarge ones and most of them in moderate or deepwater applications,deeper than 200 feet.

In deepwater mooring system applications, the gravity induced catenaryshape of the mooring line provides the basic restoring force as theoffsets occur when the vessel under environmental loading such as winds,waves and currents. At the end of the mooring line at seabed, anembedded anchor or a pile is utilized to fix the mooring line to seabed.The catenary shape of the mooring line makes the mooring line lateralstiffness much softer and more flexible comparing to a straight lineaxial stiffness of steel wires or steel chains.

In a typical chain-wire-chain configuration with an embedded anchor oran installed pile anchor at seabed, a portion of the chains is laid onthe seabed in normal conditions. When a large vessel offset occurs, thisportion of chains could be lifted off the seabed to add to catenaryshape of the mooring line. However, under any design conditions, thiscatenary mooring line can not be allowed to be a straight line or near astraight line by increasing the line tension, because the line stiffnesscould be increased significantly under a straight line configuration.

FIG. 1 illustrates one conventional chain-wire-chain mooring line systemfor an offshore moored floating barge 10. The upper portion of thechains 11 connects to a chain stopper 12 at barge deck 13. The middleportion of mooring line is a steel wire 14. The bottom portion of chains15 touches seabed 16 and connects to an embedded anchor pile 17.

In a shallow water application such as water depth less than 200 feet,the catenary shape of a conventional chain-wirechain mooring line isvery limited and the system becomes ineffective. The line could beeasily becomes a straight line as the vessel offset occurs and linetension increases. The sudden increase in line stiffness and associatedthe sudden increase in line tension could be very dangerous to themooring system. Conventional method to improve the system is to use along distance heavy chains on the seabed or to add clump weights to thechains on the seabed.

As the water depth gets shallower and shallower, the catenary mooringsystems become less and less effective. The cost of the extended lengthof the chains laid at seabed and the required clump weights becomes veryexpensive.

Non-steel synthetic mooring lines, mostly referred to Polyester lines,have been accepted recently by offshore industry. Two deepwater Sparplatforms, “Mad Dog” in 4,200 foot water depth and “Red Hawks” in 5,300foot water depth, have employed Polyester lines as mooring lines andboth were installed in Gulf of Mexico in 2004. The chain-Polyester-chainconfiguration mooring line is much lighter and much more flexible thansteel chain-wire-chain configuration. In installed condition, thePolyester line provides its restoring force mostly from its axialstiffness, not from the gravity induced catenary shape like theconventional steel mooring line.

In the application of Polyester mooring lines, there is one restrictionto be imposed: the Polyester portion of the line can not laid on theseabed to be exposed to mud or sands for an extended time. The mud orsands could get into between fibers to cause wearing damage to thePolyester lines. For deepwater applications, this restriction can beeasily satisfied. The bottom section of the chain could be designed sothat the portion of Polyester line will not touch the seabed at alltime. In shallow water applications, this restriction is very difficultto be satisfied because of the shallow water depth.

SUMMARY OF INVENTION

In one embodiment, a shallow water mooring system for large offshorefloating vessel applications is disclosed. The mooring system utilizessynthetic nonsteel lines and at least one elevation means selected froma submerged tower structure, or at least one buoyant object.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a prior art chain-wire-chain mooring system.

FIG. 2 shows a mooring system according to one embodiment of theinvention.

FIG. 3 is a side view of a mooring system according to one embodiment ofthe invention.

DETAILED DESCRIPTION

FIG. 2 illustrates the plan view of one embodiment of a mooring system.The barge 10 is moored by 12 mooring lines in three groups. A submergedtripod anchoring tower 18 is served as an elevated anchor at seabedconnected to the bottom portion of chains 15.

FIG. 3 illustrates the side view of the preferred embodiments of themooring system. The upper portion of the chains 11 connects to a chainstopper 12 at barge deck 13. The middle portion of mooring line isPolyester line 19 in two sections. Between the sections, a buoy 20 isattached with a tri-plate 21 to connect both Polyester sections 19 andthe buoy 20. The bottom portion of chains 15 connects to the elevatedanchoring tower 18. Three piles 22 are driven into the seabed throughthe three leg sleeves 23 of the tripod anchoring tower 18. Groutedconnections are applied to the connections between the piles 22 and thesleeves 23.

The length of the Polyester line for each mooring line should depend onthe required stiffness. The longer of the line, the more flexible of thePolyester line. The size of the Polyester line should depend on therequired line tension strength based on dynamic mooring analysis. Theheight of elevated anchoring tower and the size of the buoy should bebased on the mooring analysis results to keep the Polyester line off theseabed.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A shallow water mooring system for a large offshore floating vesselapplication (>300′ in vessel length) in a water depth range between 20feet and 200 feet utilizing synthetic nonsteel lines and at least oneelevation means selected from (1) the use of a submerged tower structurefixed at seabed to serve as an elevated anchor which will allow thebottom end of a mooring line connection to be at least 5 feet aboveseabed, and (2) the use of at least one buoyant object disposed at atleast one point along the length of the synthetic line.
 2. The shallowwater mooring system of claim 1 where said synthetic mooring linecomprises a Polyester line.
 3. The shallow water mooring system of claim1 wherein said submerged tower structure is a piled structure fixed tothe seabed.
 4. The shallow water mooring system of claim 1 wherein saidsubmerged tower structure at least partially protrudes above a surfaceof the water.
 5. The shallow water mooring system of claim 1 whereinsaid submerged tower structure is a gravity-based structure fixed to theseabed.
 6. The shallow water mooring system of claim 1 wherein said atleast one buoyant object comprises at least one buoy disposed at atleast one point along the length of the mooring line.
 7. The shallowwater mooring system of claim 1 wherein said at least one buoyant objectcomprises at least one buoyant foam disposed along the length of themooring line.